1. Field of the Invention
This invention relates to a pneumatic radial tire for heavy load vehicles and more particularly to a so-called combi-radial tire particularly useful for truck, bus, or the like, and including a bead portion having excellent durability.
2. Description of the Prior Art
The above mentioned kind of radial tire for heavy load vehicles is generally divided into two kinds of tires, that is, an all steel tire in which carcass and belt are composed of metal cords and a combi-tire which makes use of an organic textile cord carcass and steel cord belt. These all steel tires and combi-tire have advantages and disadvantages, respectively.
The all steel tire usually makes use of a single ply carcass. On the contrary, the combi-tire not used for passenger car but used for heavy load vehicles makes use of a number of plies. For example, if the bead portion is composed of three plies, two inner plies are wound around a bead core from the inside toward the outside thereof to form a turn-up portion, while the outer ply is wound around the bead core from the outside toward the inside thereof, thereby constituting an up-down construction.
Such a carcass ply arrangement considerably lowers the productivity of the tire at the step of forming the carcass ply. But, it is obliged to use such arrangement owing to the following reasons. That is, the combi-tire for heavy load vehicles tends to concentrate stress at the upper end of the turn-up portion of the carcass owing to the tension balance of the carcass when the inflated tire runs under load. As a result, there is a risk of separation failure being induced at the bead portion which is a vital defect to the tire.
In order to eliminate such lowered productivity, three carcass plies are wound around the bead core from the inside toward the outside thereof to form the turn-up portion. Outside these turn portions one chafer composed of metal cords is arranged and, outside such chafer is arranged an additional chafer composed of organic textile cord layer thereby forming a bead portion reinforcement.
Such bead portion reinforcement is advantageous for tire productivity, but it is impossible to completely prevent separation failure at the upper ends of the carcass turn-up portion and bead portion reinforcement, thereby shortening the durable life of the tire.